The conventional liquid metal fast breeder nuclear power reactor has an open top cylindrical vessel containing a core, and fissionable fuel elements are located in the core. By proper manipulation of control elements relative to the fuel elements, nuclear fission will take place in the vessel. Typically, molten sodium is used as a primary coolant to flow through the reactor core and over the fuel elements therein operable to pick up the heat of fission. This heat gain in the primary coolant is transferred via heat exchangers to a secondary circulating coolant (commonly water) which in turn is directed to remotely located steam turbines for generating electrical power. Primary coolant pumps circulate the sodium through the core and heat exchanger. A top horizontal deck across the open top of the reactor vessel seals the core from the atmosphere ambient the reactor. The pumps, the heat exchangers, and many control structures for the reactor frequently are carried by the deck, and they penetrate through the deck to have portions located inside of the reactor vessel and portions located outside of the reactor vessel.
The deck is supported by overlapping flange and shoulder configurations at the upper portion of the reactor vessel. The deck extends below the flange configuration as a downwardly projecting cylindrical portion that fits closely within the reactor vessel. This cylindrical portion is comprised of a cylindrical metal sleeve disposed coaxially of the reactor vessel, and of spaced generally parallel top and bottom transverse walls which interconnect at their peripherial edges with the cylindrical sleeve. Smaller cylindrical walls are also used to interconnect the top and bottom walls in the region where the components carried by the deck pass through the deck. Webs or gussets also are used to interconnect the cylindrical sleeve and walls and the transverse walls for structurally reinforcing the deck. This provides the dimensional stability and strength needed to support the pumps, heat exchangers, controls etc.
Depending upon the design and size of the reactor, the deck can be between one and possibly three meters in thickness. The lowermost portion of the deck is exposed to the primary coolant of sodium at elevated temperatures while the uppermost portion of the deck is exposed to the ambient atmosphere within the containment building and moderate temperatures. The deck is thus comprised of material to define barriers against the transfer of heat through the deck, as well as barrier to reduce the release of radiation through the deck.
Radiation barriers are generally formed by dense material such as concrete or steel, layered as sheets or blocks on one another to provide almost a solid obstruction blocking the path of escape of the radiation. These structures are frequently disposed within the deck, between the top and bottom transverse walls thereof.
Thermal barriers have frequently been carried on the underside of the bottom transverse deck wall, directly overlying and being exposed to the sodium coolant. One common construction provides using wire fabric or mesh, frequently in the form of stainless steel wool, which is housed then in panels or casings formed of telescoping metallic half boxes. The boxes are applied against the bottom face of the deck. This system is thermally effective, but the fabrication of the wire fabric or stainless steel mesh is very costly, and the labor factor required for cutting and shaping it to fit into the composite panel is also very high. Further, the wire fabric or steel wool mesh is comprised of many fine or small cross section strands that during shaping can be easily separated as shavings or the like from the bulk, that could fall into the reactor to cause a contamination problem. To minimize this possibility, extra care must be taken involving costly time and redundancy of structure. A drawback yet remains in that the half box constrains of the wire fabric or mesh provide relatively good conduction paths for locally transferring heat through the barrier, to reduce its overall effectiveness.